In all reality, if you can't pay for it in 2-4 months you shouldn't buy it. That doesn't mean you should actually pay for it in that time frame, but if it doesn't or can't generate enough profit to actually pay for itself in that short amount of time, you are probably not ready to make the investment. We lease all of our equipment to help spread out cash flow, but I won't bring in a machine if I'm not 100% confident it could pay for itself almost immediately (my rule for my company is if it can't generate the profits to pay for itself in no more than 3 months I won't buy it), regardless of how we actually buy it.
If this is your first printer and you haven't started to cultivate that market enough to justify the purchase, start selling the service and outsource it until your profits are steady over 3-6 months and are high enough that you could pay for the machine in a few months. A big mistake people make is taking the "field of dreams" approach... "if I buy it, the business will follow". This may be true for some industries, but it's not really for the sign business (yes, you may pick up some additional business when people hear you have the capability, but not nearly enough to justify the purchase).
If you bring printing in house and you are only able to generate an additional $500-$1000 per month in profits, that will cover your additional overhead (lease payment, etc.) on the equiment, but you're not making anything and you'd be better off outsourcing until you are generating at least 2-3 times that amount.